1. Field of the Invention
The invention relates to electric lamp envelopes and somewhat more particularly to glass lamp envelopes provided with a SiO.sub.2 -B.sub.2 O.sub.3 glass layer on the inner surface thereof and a method of producing such envelopes.
2. Prior Art
Energy-saving operations due to high light yields are achieved with high-pressure gas discharge electric lamps utilizing operating temperatures ranging from about 4000.degree. to 6000.degree. Kelvin in the discharge zone of such lamps. Such operating temperatures cause significant thermal and mechanical stress on the walls of such lamp envelopes. Therefore, the prior art utilizes a silica glass as the envelope material for metal/halogenide discharge lamps. The loadability or, alternatively, the life-expectancy of such lamps are limited by the aging or deterioration of the silica glass envelopes. Particularly, crystallization of silica is detrimental to the life-expectancy of envelopes and must be suppressed in silica glass envelopes. To achieve crystallization suppression, a process known as "boronization" is used whereby boron oxide is diffused into the silica glass envelope on the interior surface thereof as a crystallization inhibitor. The resultant silica glass envelope then has a SiO.sub.2 -B.sub.2 O.sub.3 layer a few .mu.m in thickness on its inner surface and such material exhibits a relatively low crystallization tendency. However, the hydroscopic nature of this layer is of great disadvantage because water bonded in or on such SiO.sub.2 -B.sub.2 O.sub.3 -layer reduces the light permeability and thereby causes an increase of the envelope temperature of the lamp. Further, water effects material transport mechanisms within the lamp in an undesirable manner. Yet further, the difference between the respective coefficients of expansion of SiO.sub.2 and SiO.sub.2 -B.sub.2 O.sub.3 glass has an unfavorable effect. This difference can cause fissures, cracks and the like to form in the SiO.sub.2 -B.sub.2 O.sub.3 layer of a boronized lamp envelope and significantly detracts from the manufacture of durable electrode seals.
Up to the present time, boronized lamp envelopes were stored in the dryest possible atmosphere before being hermetically sealed. No attention was given to a water-free state during boronization. In forming the necessary electrode seals, special un-boronized silica glass envelope sections were joined to the boronized lamp envelope sections.